Field of the Invention
The present invention concerns a magnetic resonance apparatus and method to determine a scan sequence based on a pulse response of a gradient system of the apparatus, in particular, based on a representation of the pulse response in k-space.
Description of the Prior Art
In magnetic resonance imaging, gradient fields are used to spatially encode the magnetic resonance (MR) signals that are entered into a memory organized as k-space. Spatially resolved MR images can be obtained therefrom.
The accuracy of the spatial resolution of the MR images is correlated with the accuracy with which the gradient fields that are used is known. It is frequently observed that the gradient fields designated for use deviate from gradient fields that are actually observed (gradient field errors) when the scan sequence is executed. For example, the gradient field error can occur as the result of eddy currents and/or mechanical resonances. Generally, the gradient system of the MR installation has a complicated system response between the specified or programmed gradient pulses and the gradient fields actually produced thereby.
It has been observed that gradient field errors can be particularly significant for certain scan sequences. For example, significant gradient field errors can occur in scan sequences with non-Cartesian k-space trajectories, for example spiral trajectories or trajectories with radially oriented branches. Generally, trajectories with rapid changes to the gradient fields can effect significant gradient field errors. A further example is echo planar imaging (EPI). In such cases, the gradient field error can be a function of position of the acquired data in k-space.
The gradient field error can result in artifacts such as ghosts and/or blurring in the MR images. The gradient field error may also result in reduced spatial resolution. This complicates the analysis of physical information in the MR images.
Techniques are known for reducing the gradient field error. For example, eddy current compensation can be used. A further example is retrospective correction by measuring the actual k-space trajectory. A further correction technique uses a priori knowledge on the pulse response of the gradient system as described, for example, in Vannesjö et al., “Gradient System Characterization by Impulse Response Measurements with a Dynamic Field Camera,” Magnetic Resonance in Medicine, Vol. 69, No. 2, pp. 583-593 (2013); and Vannesjö et al., “Image Reconstruction Using a Gradient Impulse Response Model for Trajectory Prediction,” Magnetic Resonance in Medicine (2015); and Vannesjö, “Characterizing and Correcting for Imperfect Field Dynamics in Magnetic Resonance Imaging,” Dissertation, Swiss Federal Institute of Technology, ETH Zurich, No. 21558 (2013). These techniques are sometimes called GIRF techniques (GIRF=gradient impulse response function).
Reference implementations of GIRF techniques have certain drawbacks and restrictions. Although a GIRF technique can often predict the gradient fields actually used with a certain degree of accuracy, the inherent characteristic remains that the gradient system tends to function as a low pass filter. This means that, often, rapid changes to gradient fields cannot be generated or can be generated only to a restricted degree. This can result in persistent gradient field errors during the emission of RF pulses, which cause inaccuracies in the manipulation of the nuclear spin magnetization. Echo paths induced by multiple RF pulses are not taken into account.